Crews were using multiple beaters, hose reel jets, main lines and compressed air foam to extinguish the fire.

Local resident Leonard Stack, 62, said: "The sky just filled with smoke. I was out cycling at the time it started and nearly choked on the smoke.

"All of a sudden you couldn't see the sun or the trees anymore.

"Because of the way the wind has been blowing the fire has spread towards the houses on Beacon Road and I wouldn't be surprised if it has burnt down some of the fences."

PC John Snelling, from Dorset police, said: "We have had three casualties with smoke inhalation taken to hospital and 40 displaced residents.

"The fire came right up to the perimeter wall of one of the properties. If you look over the wall the heath looks like the aftermath of a nuclear explosion.

"We don't yet know how the fire started but we will be working with the fire service to investigate the cause."

He said the blaze has also killed hundreds of precious reptiles tand wildlife hat inhabit the heath.

He added: "There is an area of about 1,000 square metres affected and it couldn't have come at a worse time in terms of the wildlife because of all the ground-nesting birds.

"This fire will put the heath back 25 years, it will take that long to recover."

A spokesman for Dorset Fire and Rescue Service said four members of the public were treated on scene for smoke inhalation.

One firefighter did have to go to hospital for heat exhaustion while a second received treatment for minor burns.

At the height of the fire the plume of smoke billowed about 500ft into the air.

He said: "We are carrying out an investigation but we do believe the fire might be malicious. The last fire of his magnitude was in 1976."

Keith Bacon, the area commander for Bournemouth and Poole, said: "We have got the south, east and west side of it under control but there is a 300 metre line of fire to the north that we are still dealing with.

"The vegetation on the heath is tinder dry and with the way the wind was blowing the fire spread very quickly.

"It got to about 30 metres from the back gardens of the properties on Beacon Road which is very close."

He added: "We put the fire out at that point but then the wind direction changed and it started up again and headed north west towards open ground.

"At its height there was a very large plume of smoke. Four members of the public were treated by ambulance staff at the scene for smoke inhalation.

"Unfortunately one firefighter had to go to hospital as a precautionary measure because he was suffering from extreme exhaustion, another firefighter has received medical attention due to minor burns to his arm."

After bringing it under control, firefighters were expected to be on the scene until the Friday morning.